The invention relates to a testing device for a tube forming limit curve used for the hydraulic bulging technique in the mechanical engineering technical field, comprising an upper die, a lower die and a push rod, wherein, the upper die comprises an upper connecting plate and an upper insert block; the upper connecting plate is connected to a main cylinder of a hydraulic bulging machine; the upper insert block is fixed on the upper connecting plate and positioned through a pin stop; the lower die comprises fixed blocks, a lower die set and a lower insert block; the lower die set is fixed on aworktable of the hydraulic bulging machine and adopts a symmetrical structure; both ends of the lower die set have semi-cylindrical die cavities; the lower insert block is fixed inside a lower die set die cavity; the number of the fixed blocks is two; a semi-cylindrical die cavity is arranged on each fixed block; and the two fixed blocks are fixed on both ends of the lower die set and form two cylindrical die cavities respectively with the two semi-cylindrical die cavities of the lower die set. The testing device not only can make a tube be able to deal with path distortion with linearity or quasi-linearity and reach the state of limit strain but also can make the strain ratio change between -1 and 1.